We introduce a general surface passivation mechanism for cesium lead halide perovskite materials (CsPbX 3 , X = Cl, Br, I) that is supported by a combined experimental and theoretical study of the nanocrystal surface chemistry. A variety of spectroscopic methods are employed together with ab initio calculations to identify surface halide vacancies as the predominant source of charge trapping. The number of surface traps per nanocrystal is quantified by 1 H NMR spectroscopy, and that number is consistent with a simple trapping model in which surface halide vacancies create deleterious under-coordinated lead atoms. These halide vacancies exhibit trapping behavior that differs between CsPbCl 3 , CsPbBr 3 , and CsPbI 3. Ab initio calculations suggest that introduction of anionic X-type ligands can produce trap-free bandgaps by altering the energetics of lead-based defect levels. General rules for selecting effective passivating ligand pairs are introduced by considering established principles of coordination chemistry. Introducing softer, anionic, X-type Lewis bases that target under-coordinated lead atoms results in absolute quantum yields approaching unity and monoexponential luminescence decay kinetics, thereby indicating full trap passivation. This work provides a systematic framework for preparing highly luminescent CsPbX 3 nanocrystals with variable compositions and dimensionalities, thereby improving fundamental understanding of these materials and informing future synthetic and post-synthetic efforts towards trap-free CsPbX 3 nanocrystals.

Lead halide perovskites hold promise for photonic devices, due to their superior optoelectronic properties. However, their use is limited by poor stability and toxicity. We demonstrate enhanced water and light stability of high-surface-area colloidal perovskite nanocrystals by encapsulation of colloidal CsPbBr quantum dots into matched hydrophobic macroscale polymeric matrices. This is achieved by mixing the quantum dots with presynthesized high-molecular-weight polymers. We monitor the photoluminescence quantum yield of the perovskite-polymer nanocomposite films under water-soaking for the first time, finding no change even after >4 months of continuous immersion in water. Furthermore, photostability is greatly enhanced in the macroscale polymer-encapsulated nanocrystal perovskites, which sustain >10 absorption events per quantum dot prior to photodegradation, a significant threshold for potential device use. Control of the quantum dot shape in these thin-film polymer composite enables color tunability via strong quantum-confinement in nanoplates and significant room temperature polarized emission from perovskite nanowires. Not only does the high-molecular-weight polymer protect the perovskites from the environment but also no escaped lead was detected in water that was in contact with the encapsulated perovskites for months. Our ligand-passivated perovskite-macroscale polymer composites provide a robust platform for diverse photonic applications.

A premelted layer of water wets the surface of ice at temperatures below the melting temperature. Experiments suggest that this quasi-liquid layer may play an important role in the nucleation of clathrate hydrates from ice. Nevertheless, the structure of the quasi-liquid layer of ice in the presence of methane or other clathrate-forming gases has not yet been elucidated. In this work, we perform large-scale molecular dynamic simulations with a coarse-grained molecular model to investigate the properties of the quasi-liquid layer of ice in the presence of methane gas under pressure. We characterize the structure and thickness of the ice/methane and ice/vacuum interfaces, and the solubility of methane in the premelted layer as a function of temperature. We find that the width of the quasiliquid layer fluctuates between 5 and 45 Å in the presence of a methane-like solute at temperatures within 1 K of the melting point. The width of the quasi-liquid layer of ice at temperatures lower than 270 K is less than the diameter of a water dodecahedron, the smallest cage that constitutes the clathrates. The simulations indicate that, when the premelting layer is wider than 10 Å, the structure of water and solubility of methane in the center of the quasi-liquid layer are the same as in bulk liquid water at the same temperature. These results are relevant for understanding the mechanism of formation of methane hydrate clathrates from ice.